1. Field of the Invention
The invention relates to synthetically generated displays for aircraft flight instrumentation, and more particularly to a hybrid cathode ray tube display using digitally generated rasters and stroke vectors which superimposes stroke or raster symbols over the vector display in accordance with a desired priority.
2. Description of the Prior Art
In the use of electronic display systems for aircraft instruments it has become increasingly important to devise methods for presenting the information to the flight crew in a clear, uncluttered manner. A conventional hybrid display system includes a stroke vector generator and a raster generator which supply alternately and sequentially a single CRT with a picture that includes both raster and stroke information. Stroke written CRT displays trace the shape of figures to be presented by deflecting the electron beam in a manner which connects a successive sequence of strokes or vectors, which may be straight or curved. In a raster system the beam is caused to trace a repetitive pattern of parallel scan lines and the information is presented by intensity modulating the electron beam at the appropriate points along each line.
As the size of cathode ray tube (CRT) displays increases, more symbology is placed on an individual indicator. Correspondingly, as the quantity and the complexity of the symbology increases, the risk of misinterpretation of data due to clutter and overlapping may increase. A key objective of the electronic CRT display technique is the assignment of priority of symbology to defined areas of the screen. A symbol of importance, when it intersects one of less importance, should appear to be on a plane closer to the viewer. The less important symbol should disappear behind it. It is also desired that this effect be attained even if both symbols are allowed to move. This minimizes any conflict of data presentation and is particularly effective, for example, as compared to the clutter and parallax of the flight director command cue presentation that is typical of conventional electromechanical attitude director indicators.
Prior art schemes for priority implementation generally employed a combination of stroke vectors for generating numerical data and index lines and raster generators for generating other symbology and background colors. One technique is a "cell" storage technique, which divides the display into a matrix of incremental display cell areas of the display screen. This approach is shown in U.S. Pat. No. 4,070,662, issued to Parm L. Narveson and assigned to the assignee of the present invention. A symbol memory provides for storing a plurality of patterns and symbols which can be selectively fetched to form a display picture. This technique is adequate for static priority areas, but is very cumbersome when the priority areas are changing. To produce a display pattern in which lower priority areas are masked, the display processor must first determine the pattern of on and off picture elements for each defined cell in the display. It must then generate the proper symbol representing the cell's pattern or choose from a predetermined library of symbols, and place that pattern at the proper cell location on the display. This imposes a considerable time-consuming load on the display processor, and is particularly difficult when moving symbology is necessary. Such software intensive techniques have a primary disadvantage of using large amounts of valuable computer time in a real-time system, where processing time is critical. Thus, although such symbology can be accomplished, it can be done only to a limited extent in practice in an aircraft instrumentation display and may require a significant amount of processor time to calculate the appropriate cell and symbol definitions.
A second approach is the full-field memory or bit-mapped technique, where each resolution element or picture element of the display is defined by a group of memory bits corresponding to the respective individual picture elements on the display screen. A picture is loaded into memory from a computer or other source of digital instructions and the entire memory is read out sequentially in synchronism with digital circuitry generating a raster. An image is produced by specifically setting, for each picture element, the color and priority desired by writing the appropriate data into the full-field memory. In readout, serial digital memory output works are converted to analog form and then transmitted to the CRT display for each frame refresh cycle. From a hardware standpoint, this approach is unattractive because of the size of the required support circuitry and processing time. Systems using a full-field memory typically provide loading of the memory with a processor. The number of storage elements in the full-field memory is the product of the vertical and horizontal resolution elements. For a display mask resolution of 256 lines by 512 pixels (picture elements) per line the number of memory bits required is 131,072. If color information is encoded, additional memory is required to specify a color. The time required for the processor to calculate the image pattern and to store so many elements in the full-field memory is considerable and may impose unacceptable restrictions on the display update rate and other required processor tasks.
The present invention utilizes the repetitive nature inherent in a conventional raster symbol generator scan to provide an apparatus for loading a stroke priority full-field memory without requiring access to extensive processor time and permits efficient generator of dynamic stroke priority areas. The circuits that are disclosed herein permit masking of the stroke vector symbology in accordance with the desired priority, and optional superposition of stroke vector and raster scan displays.